DouKaku?

package challenge120;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Challenge120 {

    private final static int LIM = 1 << 20; // 1<<20 = 2^20

    private final static Map<Long, Integer> CACHE = new HashMap<Long, Integer>();

    public static void main(final String[] args) {

        final long start = System.currentTimeMillis();

        int fnMax = 0;
        final List<Integer> fnMaxN = new ArrayList<Integer>();

        for (int n = 1; n <= LIM; n++) {
            final int fn = f(n);
            if (fn > fnMax) {
                fnMax = fn;
                fnMaxN.clear();
                fnMaxN.add(n);
            } else if (fn == fnMax) {
                fnMaxN.add(n);
            }
        }
        out(fnMax, fnMaxN);
        System.out.println("Time: " + (System.currentTimeMillis() - start)
                + "[ms]");
    }

    private static int f(final int n) {
        int fn = 0;
        for (long nTemp = n; nTemp != 1;) {
            final Integer cached = CACHE.get(nTemp);
            if (cached == null) {
                nTemp = (nTemp % 2 == 0) ? nTemp / 2 : nTemp * 3 + 1;
                fn++;
            } else {
                fn += cached;
                break;
            }
        }
        CACHE.put(Long.valueOf(n), fn);
        return fn;
    }

    private static void out(final int maxN, final List<Integer> fnMaxN) {

        System.out.println("Max(f(n)) = " + maxN);
        System.out.println("n =");
        for (final int n : fnMaxN) {
            System.out.println("  " + n);
        }
    }
}